1. Field of the Invention
The present invention relates to a thin-film magnetic head that incorporates a magnetoresistive element for reading a magnetic signal and a method of manufacturing such a thin-film magnetic head, a head gimbal assembly and a hard disk drive incorporating the thin-film magnetic head.
2. Description of the Related Art
Performance improvements in thin-film magnetic heads have been sought as a real recording density of hard disk drives has increased. Such thin-film magnetic heads include composite thin-film magnetic heads that have been widely used. A composite head is made of a layered structure including a read (reproducing) head having a magnetoresistive element (that may be hereinafter called an MR element) for reading and a write (recording) head having an induction-type electromagnetic transducer for writing, the read head and the write head being stacked on a substrate.
MR elements include: an AMR element that utilizes the anisotropic magnetoresistive effect; a GMR element that utilizes the giant magnetoresistive effect; and a TMR element that utilizes the tunnel magnetoresistive effect.
Read heads that exhibit a high sensitivity and a high output are required. Read heads that meet these requirements are GMR heads incorporating spin-valve GMR elements. Such GMR heads have been mass-produced.
In general, a spin-valve GMR element incorporates: a nonmagnetic layer having two surfaces that face toward opposite directions; a first ferromagnetic layer that is located adjacent to one of the surfaces of the nonmagnetic layer; a second ferromagnetic layer that is located adjacent to the other of the surfaces of the nonmagnetic layer; and an antiferromagnetic layer that is located adjacent to one of surfaces of the second ferromagnetic layer that is farther from the nonmagnetic layer. The first ferromagnetic layer is a layer in which the direction of magnetization varies in response to a signal magnetic field, and is called a free layer. The second ferromagnetic layer is a layer in which the direction of magnetization is fixed by the magnetic field produced from the antiferromagnetic layer, and is called a pinned layer.
Another characteristic required for read heads is a small Barkhausen noise. Barkhausen noise results from transition of a domain wall of a magnetic domain of an MR element. If Barkhausen noise occurs, an abrupt variation in output results, which induces a reduction in signal-to-noise ratio (hereinafter called S/N ratio) and an increase in error rate.
To reduce Barkhausen noise, a bias magnetic field in the longitudinal direction (that may be hereinafter called a longitudinal bias field) is applied to the MR element. To apply the longitudinal bias field to the MR element, bias field applying layers may be provided on both sides of the MR element, for example. Each of the bias field applying layers is made of a hard magnetic layer or a laminate of a ferromagnetic layer and an antiferromagnetic layer, for example.
In a read head in which bias field applying layers are provided on both sides of the MR element, two conductive layers for feeding a current used for magnetic signal detection (that may be hereinafter called a sense current) to the MR element are located to touch the bias field applying layers.
It is known that, when the bias field applying layers are located on both sides of the MR element, regions that may be hereinafter called dead regions are created near ends of the MR element that are adjacent to the bias field applying layers. In these regions the magnetic field produced from the bias field applying layers fixes the direction of magnetization, and sensing of a signal magnetic field is thereby prevented.
Consequently, if the conductive layers are located so as not to overlap the MR element, a sense current passes through the dead regions. The output of the read head is thereby reduced.
To solve this problem, the conductive layers are located to overlap the MR element.
It is possible to reduce Barkhausen noise while a reduction in output of the read head is prevented, if the read head has a structure in which the bias field applying layers are located on both sides of the MR element, and the conductive layers overlap the MR element, as described above. Such a structure is hereinafter called an overlapping conductive layer structure.
As is described in Published Unexamined Japanese Patent Application (KOKAI) Heisei 6-180825 (1994), in order to increase an S/N ratio of a read head, it is preferable to lower the resistance of the entire read head including the MR element and the conductive layers.
Methods for lowering the resistance of the conductive layers include a method that increases a cross-sectional area of the conductive layers by increasing the thickness of the conductive layers, and a method that uses a material having a small resistivity to make the conductive layers. However, increasing the thickness of the conductive layers has a process limitation. Hence, in order to achieve a read head with satisfactory characteristics, it is necessary to use a material having a small resistivity to make the conductive layers.
Generally, the conductive layers are made of a layer of gold (Au), which is a low resistance material, or a laminate of an Au layer and another metal layer. Alternatively, the conductive layers may be made of a low resistance material other than Au. For example, the conductive layers may be made of a laminate of a TiW layer and a Ta layer, or of a Cu layer.
A thin-film magnetic head has a medium facing surface that faces toward a recording medium. An end of each of the MR element and the conductive layers is exposed in the medium facing surface. The medium facing surface is subjected to lapping during the fabrication process of the thin-film magnetic head.
Au, which is used as the material for the conductive layers, has an excellent resistance to corrosion but has a low hardness. For this reason, when the conductive layers are made of an Au layer, or a laminate of an Au layer and another metal layer, there are two problems as follows. A first problem is that, during lapping of the medium facing surface, the conductive layers extend in the medium facing surface and adhere to the MR element. This causes a resistance value of the MR element to change, which results in variations in the characteristics of the read head. A second problem is that lapping of the medium facing surface causes the conductive layers to be abraded more than the other layers, which results in a difference in level between each of the conductive layers and the other layers in the medium facing surface, with the end of each of the conductive layers recessed from the ends of the other layers in the medium facing surface. When the difference in level is great, even if the medium facing surface is covered with a protection film of diamond-like carbon (DLC) or the like, a gap is produced between the protection film and the ends of the conductive layers. As a result, corrosion may occur at the gap portion and spread toward the MR element.
On the other hand, conductive layers made of a laminate of a TiW layer and a Ta layer have greater resistivity compared with the conductive layers made of an Au layer. Conductive layers made of a Cu layer have smaller resistivity compared with the conductive layers made of an Au layer, but they are vulnerable to oxidation and therefore have poorer resistance to corrosion.
Published Unexamined Japanese Patent Application (KOKAI) Heisei 6-180825 (1994) discloses AuNi as an example of materials for the conductive layers. However, this publication fails to specify preferable resistivity and preferable hardness of AuNi.